The present invention relates to microelectronic packages and specifically relates to low cost, compliant packages for high input/output and fine pitch microelectronic elements.
Complex microelectronic devices such as semiconductor chips typically require numerous connections to other electronic components. For example, a complex device including a semiconductor chip may require hundreds of electrical connections between the chip and one or more external devices. These electrical connections may be made using several alternative methods, including wire bonding, tape automated bonding and flip-chip bonding. Each of these techniques presents various problems including difficulty in testing the chip after bonding, long lead lengths, large areas occupied by the chip on a microelectronic assembly, and fatigue of the connections due to expansion and contraction of the chip and substrate during thermal cycling.
When a semiconductor chip is mounted to a circuit board, thermal expansion and contraction of the chip and circuit board can cause the contacts on the chip to move relative to contacts on the circuit board. This movement can occur during operation of the device or during manufacturing operations (e.g. when soldering the chip to the circuit board). Thus, in many microelectronic devices, in order to minimize the effects of thermal cycling, it is desirable to provide an electrical connection between components that can accommodate relative movement between the components.
One structure that has been used to successfully address thermal cycling problems is commonly referred to as a connection component such as the structures disclosed in certain preferred embodiments of commonly assigned U.S. Pat. Nos. 5,148,265, 5,148,266 and 5,455,390, the disclosures of which are hereby incorporated by reference herein. Connection components typically include a flexible sheet having a plurality of terminals thereon and flexible leads that are used to electrically interconnect the terminals with contacts on a microelectronic element, such as a semiconductor chip. The flexible leads permit thermal expansion of the microelectronic element and connection component while maintaining the electrical connection between therebetween. The terminals of the connection component may be used to test the package, and/or permanently attach the package to another microelectronic element, such as a printed circuit board. A compliant layer may be disposed between the microelectronic element and the connection component. The compliant layer typically encapsulates the leads and facilitates connection of the terminals to a test device and/or to the final electronic assembly by compensating for variations in component flatness and the heights of the terminals.
In certain preferred embodiments of commonly assigned U.S. Pat. No. 5,518,964 (xe2x80x9cthe ""964 patentxe2x80x9d), the disclosure of which is hereby incorporated by reference herein, a microelectronic package is made by first connecting flexible leads between microelectronic elements, such as a chip and a connection component, and then moving the elements away from one another through a predetermined displacement so as to bend the leads. The leads may have first ends permanently attached to the connection component and second ends releasably attached to the connection component. During assembly, the connection component may be juxtaposed with a semiconductor chip having contacts and the second ends of the leads may be bonded to the contacts on the chip. Following bonding, the connection component and chip are moved away from one another, thereby vertically extending the leads. During or after movement, a curable liquid material, such as a silicone elastomer, may be introduced between the elements. The curable material may be cured, such as by using heat, to form a compliant dielectric layer surrounding the leads. The resulting semiconductor chip package has terminals on the connection component which are electrically connected to the contacts on the chip, but which can move relative to the chip so as to compensate for thermal expansion and contraction of the elements during thermal cycling. The package may be mounted to a circuit board by solder-bonding the terminals of the connection component to conductive pads on the circuit board.
In other embodiments of the ""964 patent, the package-forming process can be conducted on a wafer scale, so that all of the semiconductor chips in a wafer may be simultaneously connected to connection components. The wafer and connection components may then be moved away from one another so as to vertically extend all of the leads of the wafer in a single step. The resulting package is severed to provide individual units, each including one or more chips electrically interconnected with a portion of a dielectric body. The above-described flexible leads may be formed on the chip or wafer, rather than on the dielectric body. In further embodiments of the ""964 patent, a dielectric body having terminals and leads is connected to terminal structures on a temporary sheet. The temporary sheet and dielectric body are moved away from one another so as to vertically extend the leads, and a curable liquid material is introduced around the leads and cured so as to form a compliant layer between the temporary sheet and the dielectric body. The temporary sheet is removed, leaving tip ends of the terminal structures projecting from a surface of the compliant layer. Such a component, commonly referred to as a connection component, may be used between two other components.
In certain preferred embodiments of commonly assigned U.S. Pat. No. 5,688,716, the disclosure of which is hereby incorporated by reference herein, a microelectronic package includes a chip and a package element, such as a heat sink. The chip has contacts electrically interconnected with terminals on a dielectric element, such as a sheet or plate. The dielectric element and chip are then moved away from one another to vertically extend the leads, and a curable liquid material is injected between the package element and dielectric element and around the leads. The dielectric element and the package element extend outwardly beyond the edges of the chip and physically protect the chip.
In certain preferred embodiments of commonly assigned U.S. Pat. No. 6,117,694, the disclosure of which is hereby incorporated by reference herein, a microelectronic package is made by connecting leads between a pair of microelectronic elements and then moving the elements away from one another so as to vertically extend the leads. After vertically extending the leads, a curable encapsulant may be injected between the microelectronic elements. The encapsulant may be injected under pressure for both moving the microelectronic elements away from one another and vertically extending the leads. Alternatively, the leads may be formed by retaining the microelectronic elements against respective platens by vacuum, and then moving the platens away from one another for vertically extending the leads. A curable liquid encapsulant may be injected while the platens maintain the microelectronic elements in their displaced positions.
Despite these and other advances in the art, still further improvements would be desirable. Specifically, there is a need for improved chip packages having high input/output or fine pitch contacts that may be made more easily and more economically.
A method of making a compliant microelectronic package preferably includes providing a first substrate having a top surface. The first substrate is preferably comprised of a metal or ceramic material. In preferred embodiments, the first substrate is a heat spreader, preferably made of metal or other thermally conductive materials such as aluminum nitride. A second substrate is attached atop the first substrate. The second substrate preferably includes a top surface having a plurality of conductive pads, a bottom surface remote therefrom and an opening extending between the top and bottom surfaces of the second substrate. After the second substrate has been attached to the first substrate, the bottom surface of the second substrate preferably confronts the top surface of the first substrate. The second substrate is preferably attached to the first substrate using an adhesive such as a low expansion adhesive or a non-expansion adhesive. The adhesive may be thermally conductive. In certain preferred embodiments, the first and second substrates have coefficients of thermal expansion that are substantially similar to one another. Although not limited by any particular theory of operation, it is believed that CTE matched substrates will expand and contract at similar rates during operation, thereby minimizing stresses and strains on any electrical interconnections (e.g. leads, wires) therebetween.
The microelectronic element, which is preferably a semiconductor chip having a front contact bearing face and a back face remote therefrom, is placed in the opening of the second substrate. The back face of the microelectronic element may be secured to the top surface of first substrate by using an adhesive. After the microelectronic element has been secured to the first substrate, the front face of the microelectronic element and the top surface of the second substrate are preferably substantially coplanar with one another. The microelectronic element preferably includes semiconductor chips, semiconductor wafers, packaged semiconductor chips and packaged semiconductor wafers, with semiconductor chips and semiconductor wafers being particularly preferred. In other embodiments, the first and second substrates may be replaced with a single, unitary substrate having a cavity for receiving the microelectronic element.
The microelectronic element is electrically interconnected with the conductive pads of the second substrate by, for example, using a wire bonding tool to attached the first ends of conductive wires to the contacts of the microelectronic element and second ends of the conductive wires to the conductive pads of the second substrate. A protective coating may then be provided over at least a portion of the wire bonds to, inter alia, prevent short circuits. In certain preferred embodiments, the bonded conductive wires are flush with the front face of the microelectronic element and the top surface of the second substrate. Although wire bonding is the preferred method for interconnecting the contacts and the conductive pads, other methods known in the art for creating electrical interconnections may also be used.
A dielectric sheet, such as a flexible dielectric film, may be juxtaposed with the top surface of the second substrate. In preferred embodiments, the dielectric sheet includes a flexible polymeric sheet. The dielectric sheet may be flexible or rigid. The dielectric sheet may be a polymeric dielectric sheet and more preferably is a flexible, polymeric dielectric sheet. The dielectric sheet may have conductive elements exposed at the second surface. In embodiments where the dielectric sheet is rigid, the dielectric sheet may be a ceramic plate, a FR4 or FR5 or bismaleimide triazine board (BT) or a multi-layer substrate. In embodiments where the dielectric sheet is flexible, the dielectric sheet is preferably polymeric, the preferred polymeric sheet including a polyimide.
The dielectric sheet preferably includes conductive leads having first ends permanently attached to the dielectric sheet and second ends releasably attached to the dielectric sheet. The dielectric sheet preferably has a first surface including the conductive leads extending over the first surface and terminals accessible at the second surface thereof. The terminals are desirably electrically interconnected with the first ends of the leads. After the dielectric sheet has been juxtaposed with the front face of the microelectronic element, the releasable ends of the leads are attached to the conductive pads of the second substrate for electrically interconnecting the leads with the contacts of the microelectronic element. As a result, the contacts of the microelectronic element are electrically interconnected with the terminals accessible at the second surface of the dielectric sheet. The releasable ends of the leads may be attached to the conductive pads using a conductive paste or a conductive adhesive. In other embodiments, the releasably ends of the leads are not attached to the substrate. In still other embodiments, a gap is present between the releasable ends of the leads and the dielectric substrate and a small spot of dielectric material may span the gap, the small spot connecting the releasable end to the substrate.
The dielectric film and the second substrate are then moved away from one another in a controlled manner so as to move the second ends of the leads away from the dielectric film, and to vertically extend the leads between the second substrate and the dielectric film. In certain preferred embodiments, the moving step includes introducing a liquid material between the dielectric film and the second substrate. As the liquid material is introduced, the curable liquid material forces the dielectric film away from the second substrate. The curable material is preferably a material having a low coefficient of thermal expansion. The liquid material may be cured so as to form a compliant layer between the dielectric film and the second substrate and microelectronic element. The curable liquid material may be cured at room temperature or by using energy or heat. Fusible conductive masses, such as solder balls, may be attached to the terminals of the dielectric film so that the assembly may be readily interconnected with the contacts of another component.
In one preferred embodiment, the dielectric film and the second substrate are moved relative to one another through a predetermined displacement so that the dielectric film moves with both a vertical component of motion away from the second substrate and a horizontal component of motion parallel to the second substrate, wherein the conductive leads are bent into a substantially S-shape configuration. In one particular preferred embodiment, the dielectric film and second substrate are moved away from one another by attaching a top platen to the dielectric film and a bottom platen to the first substrate, drawing a vacuum through the first platen so as to adhere the dielectric film to the first platen and drawing a vacuum through the second platen so as to adhere the first substrate to the second platen. While maintaining the vacuum through the first and second platens, the first and second platens are moved away from one another. As the first and second platens move away from one another, in turn, the dielectric film and second substrate move away from one another. During this time, the first releasable ends of the leads are released or peeled away from the first surface of the dielectric film and bent into a substantially S-shape orientation.
Although the present invention is not limited by any particular theory of operation, it is believed that mounting the back face of a microelectronic element to a top surface of a first substrate using an adhesive, and then using a wire bonding tool to electrically interconnect the contacts of the microelectronic element with the conductive pads of a second substrate avoids the need for costly and time consuming accurate mounting of the microelectronic element relative to the first and second substrates. Furthermore, such a package may be made rather simply and at low cost and use a gang lead forming process for electrically interconnecting the microelectronic element with terminals of a dielectric film, wherein the leads extend between the conductive pads of the second substrate and the terminals of the dielectric film. Thus, the present invention provides improved methods for manufacturing microelectronic assemblies having high input/output and fine pitch contacts using economical materials. The present invention also avoids the need for precise chip mounting tools.
In other preferred embodiments of the present invention, a method of making compliant microelectronic packages includes providing a first substrate having a top surface, and providing a second substrate having a top surface with a plurality of conductive pads, a bottom surface, and a plurality of openings extending between the top and bottom surfaces. The second substrate is attached to the first substrate so that the bottom of the second substrate confronts the top of the first substrate. Microelectronic elements are then disposed in the plurality of openings of the second substrate, each microelectronic element having a front face with contacts and a back face remote therefrom, whereby the back face of each microelectronic element confronts the top surface of the first substrate. The contacts of the microelectronic elements are then electrically interconnected with the conductive pads of the second substrate. One preferred method includes using a wire bonding tool to form conductive wire bonds between the contacts and the conductive pads. A dielectric film having leads is then juxtaposed with the second substrate and the microelectronic elements, the conductive leads having first ends permanently attached to the dielectric film and the second ends releasably attached to the dielectric film. The second ends of the leads are then attached to the conductive pads of the second substrate so as to electrically interconnect the leads with the contacts of the microelectronic elements. The dielectric film and the second substrate are moved away from one another so as to vertically extend the leads between the dielectric film and the second substrate. A curable liquid encapsulant, may be disposed between the dielectric film and the second substrate so as to provide a compliant layer therebetween. The resulting assembly may be severed to provide individual microelectronic packages including one or more microelectronic elements and a portion of the dielectric substrate.
In accordance with yet another preferred embodiment of the present invention, a microelectronic package includes a first substrate having a top surface, and a second substrate having a top surface including a plurality of conductive pads, a bottom surface remote therefrom and an opening extending between the top and bottom surfaces, whereby the second substrate is attached to the first substrate so that the bottom surface of the second substrate confronts the top surface of the first substrate. A microelectronic element having a front face with contacts and a back face remote therefrom is disposed in the opening of the second substrate and attached to the first substrate, the back face of the microelectronic element confronting the top surface of the first substrate. The conductive pads of the second substrate are preferably electrically interconnected with the contacts of the microelectronic element, such as by using conductive wires.
The package also preferably includes a dielectric sheet overlying the second substrate and the microelectronic element disposed within the opening of the second substrate, the dielectric film including leads vertically extended between the second substrate and the dielectric film for electrically interconnecting the microelectronic element and the dielectric sheet. The vertically extended leads may have an S-shape so that the leads flex and bend during thermal cycling of the components. The package also preferably includes a compliant layer between the dielectric sheet and the second substrate and around the leads. The dielectric film may have terminals exposed at a top surface thereof, the terminals being electrically interconnected with the leads. Masses of fusible conductive material may be deposited atop the terminals. The masses of fusible conductive material may include material selected from the group consisting of tin, lead and combinations thereof.
These and other preferred embodiments of the present invention will be described in more detail below.